This invention relates to a servo motor system of a transportation line for conveying an article such as a workpiece for assembly or machining, and more particularly to a system with a predetermined travel distance having a low positioning accuracy such as greater than 0.1 mm.
Two typical conventional transportation servo motor systems are the incremental system, as shown by FIG. 5A, and the absolute system, as shown by FIG. 5B. In the incremental system, a detector 55 of a servo motor 54 outputs a signal to a counter 52 and amplifier 53 of the motor driver 58 indicative of an incremental distance the article is moved by the motor 54, but not the absolute position of the article, The coordinate system of the movable space of the article is defined in a controller 51 when the article is positioned at a definite point in the transportation line, which is defined as the origin of the coordinate system. In the absolute system, however, a detector 67 outputs a signal indicative of an absolute position of a servo motor 64 in the whole movable space of the article to a motor driver 68. The coordinate system is defined in controller 66 by receiving the absolute position signal from the detector 67. Namely, in the absolute system, the servo motor system can perform normal operation immediately after the power to the system is turned on, because the absolute position of the article, wherever it is, can be always known by the controller 66. In the incremental system, however, normal operation of the servo motor system cannot be started before the article is first brought to the origin of the coordinate system after the power is first turned on.
But the absolute system has a drawback in that the detector 67 must be complex, and the increase in the amount of data to be processed in the controller 66 and the amplifier 63 increases their cost. Elongated wiring of the whole system also increases the total cost of the system.
The basic requirement for the servo motor system in general, is to drive the servo motor precisely in accordance with the controlled amount, however complicated it is. For that purpose, feedback data, such as the position (P), velocity (V) and current (I), is continuously fed to the motor driver 58 or 68, because the driver 58 or 68 directly controls the movement of the motor 54 or 64. On the other hand, in most cases, the feedback data is not always fed to the controller 51 or 66, because in practice it is difficult to externally control the motor 54 or 64 by the controller 51 or 66 when the minimum incremental moving distance is very small. For example, the minimum incremental moving distance corresponding to one pulse is as small as 1.0 to 0.1 micrometer in the control of numerically controlled (NC) machine tools.
Though it is shown in FIG. 5B that the controller 66 receives feedback data of the absolute position from the detector 67, it is done only at the start of the operation in order to establish the coordinate system and the controller 66 does not receive the feedback data afterwards. It can be said, therefore, that the control of the motor 54 or 64 by the incremental system is essential to the absolute system as well as the incremental servo motor system.
After the development of the AC servo motor, the servo motor has been widely applied and they are now often used in transportation systems. This is because the quick and accurate movement of the servo motor is quite suitable for driving a transportation line.
In a transportation system for conveying articles having dimensions in the order of meters, the positioning accuracy in the order of millimeters, such as greater than 0.1 mm, is sufficient. This is greatly different from the accuracy required in the NC machine tools by 10.sup.3 to 10.sup.4 orders of magnitude.
Safety is always most important in designing a system and it is especially important for the transportation system because the conveyed articles are usually big, the running speed is rather high, and the traveling distance is long. It is, therefore, necessary to always recognize the position of the article. In this respect, the absolute servo motor system is preferable to the incremental system since the incremental system requires excessive movement for bringing the article to the origin of the coordinate system.
As explained so far, no prior art servo motor system, neither the incremental system nor the absolute system, are suitable for the transportation system in which the travel distance is predetermined and the positioning accuracy is low (greater than 0.1 mm).
There already exists a servo motor system appropriate for transportation. The servo motor system includes: an incremental controller; an incremental detector; a first external signal generator for providing the servo motor with a low speed signal upon detecting the article; and a second external signal generator for providing the servo motor with a stop signal upon detecting the article. In this system, the article is conveyed, utilizing the incremental controller, and the incremental detector normally as an incremental servo motor system, but, at required times, the incremental controller is corrected by signals from the first and second external signal generators.
In this system, however, the moving distance of the article is fixed. When the moving distance is desired to be changed, the control distance should be changed in the controller. For every stop of the article at the predetermined destination point where the second external signal generator is located, the normal control by the controller is stopped and a clear pulse is input in the motor driver. When the clear pulse is received, the servo motor and the mechanical system tend to jerk and an over-shoot occurs. Alternatively, a speed adjustment at low speed running and a deliberate adjustment of the position of the external signal generator is required in order to avoid such a jerk and over-shoot.